Diapedesis is a critical step in the inflammatory response in which leukocytes migrate across endothelial cells (EC) out of the circulation and into inflamed tissues. We have found that the molecule PECAM (CD31), expressed at the borders of endothelial cells and on leukocytes, is a critical regulator of diapedesis. We also described a novel PECAM-containing membrane compartment, the lateral border recycling compartment (LBRC), a reticulum of interconnected vesicles in continuity with the plasma membrane at the borders of endothelial cells. During diapedesis, the LBRC moves to surround the transmigrating leukocyte as it moves through the endothelial cell border. Over the past funding period, we demonstrated that this targeted recycling was necessary for diapedesis to proceed. Furthermore, the critical role of PECAM in transmigration relates to its presence in the LBRC. Mutations that block its ability to enter the LBRC and function in targeted recycling block diapedesis. The aims of this application are to investigate how PECAM and the LBRC regulate diapedesis, with special attention to the role of PECAM in LBRC function. During diapedesis, the junctional adhesion molecule VE-cadherin is removed from the membrane at the site of diapedesis, while the PECAM and other molecules from the LBRC are recruited. Blocking either VE-cadherin removal or targeted recycling of the LBRC blocks diapedesis. Aim 1 will test the hypothesis that removal of VE-cadherin is functionally linked to targeted recycling of the LBRC. Targeted recycling requires intact microtubules and microtubule molecular motor activity. Aim 2 will test the hypothesis that a specific microtubule molecular motor (kinesin) is required for targeted recycling of the LBRC and will identify that kinesin (or kinesins if there is more than one involved to a significant extent.) Mutations in the cytoplasmic tail of PECAM that interfere with its ability to support transmigration also interfere with its ability to enter and leave the LBRC. Since the signaling properties of the cytoplasmic tail were not affected, we believe that the effects of this mutation on transmigration were due to the effect on PECAM's ability to function in the LBRC. Aim 3 will test the hypothesis that the cytoplasmic tail of PECAM contains a localization sequence critical for entry into the LBRC. While we are dissecting the role of PECAM in diapedesis in these in vitro studies, we will study the role of PECAM in diapedesis in vivo. Aim 4 will use intravital microscopy to examine the relationship of leukocytes with endothelial cell border in real time in living mice. We will first test the hypothesis that blocking PECAM arrests leukocyte transmigration in vivo the same site as it does in vitro (at the luminal side of the endothelial cell border in all mouse strains exceptC57Bl/6). We will next study the route of transmigration taken by leukocytes under PECAM-independent inflammatory conditions. The results of these studies will contribute significantly to our understanding of the regulation of diapedesis and provide new targets for anti-inflammatory therapy.